To date, fluid viscosity is obtained by mechanical means. Fluids are sheared, and the resistance against shear is related to viscosity. All mechanical methods have in common that the measurement process is time-consuming and requires relatively large amounts of fluid. The measurement apparatus needs scrupulous cleaning, which increases time requirements for measurement series. When measuring biofluids, which contain large amounts of proteins and/or colloids, protein deposition at the instrument surface introduces errors, as does the protein interaction at the liquid/air interface. Fluorescent molecular rotors, molecules that change their fluorescence quantum yield with the viscosity of the environment, allow viscosity measurements in biofluids without the above disadvantages. A viscometer is proposed in this development grant that allows viscosity measurements of biofluids based on fluorescent molecular rotors. The first phase of this grant addresses specific issues such as rotor-protein interaction, measurability of various colloids, fluid turbidity and absorption. It will be attempted to find a specific molecule and a specific measurement technique that allows viscosity measurements with a precision that exceeds that of standard viscometers. During the second phase, two prototypes of fluorescence-based viscometers will be developed. One aims at high accuracy for laboratory usage, and the second will be a fast, low-volume battery-powered field-usable device. Both devices will have applications in research areas that involve blood plasma, lymphatic fluid and interstitial fluid viscosity. One additional application is the monitoring of blood replacement using high-viscosity plasma expanders in trauma medicine.